Parametric transmit waveform generator for medical ultrasound imaging system

ABSTRACT

A medical diagnostic ultrasonic imaging system includes a transmit waveform generator that uses stored parameters to completely define an arbitrarily complex transmit waveform. Preferably, the stored parameters define an envelope function and a modulation function in a piecewise fashion using a number of sets of quadratic parameters. These quadratic parameters are used to calculate the desired envelope function and modulation function in the log domain, and the envelope and modulation functions are combined in the log domain and then converted to the linear domain. Multiple separate transmit waveforms may be combined in a single channel, and individual channels may be combined prior to application to the transducer elements.

BACKGROUND

The present invention relates to medical diagnostic ultrasonic imagingsystems, and in particular to digital transmit waveform generatorsadapted for such systems.

In the prior art, digital transmit beamformers are known that use amemory such as a RAM to store a sampled version of the desired transmitwaveform envelope. The data stored in RAM can be a complex basebandenvelope sampled at the Nyquist frequency. See Cole, U.S. Pat. No.5,675,554, assigned to the assignee of this invention. In this case,signal processing techniques are then used to interpolate, filter, andmodulate the envelope to form the desired ultrasonic transmit waveform.In some cases, multiple simultaneous transmit beams are generated inreal time in a time-interleaved manner. See the above-identified Colepatent. As another alternative, the desired ultrasonic transmit waveformcan be stored directly in RAM.

The methods described above require a memory size that increaseslinearly with the time duration of the transmit waveform. For longtransmit waveforms such as coded excitation transmit pulses, the numberof samples stored in memory can exceed currently available RAM sizes.For a given RAM size, the number of samples required for each transmitwaveform limits the number of concurrent RAM transmit waveforms. In somecases, this can limit the number of distinct transmit waveforms perbeam, or may require reloading the RAM on a line-by-line basis, whichmay adversely affect the frame rate.

The process of interpolating, filtering and modulating a Nyquist-sampledbaseband signal can in some cases limit the final bandwidth andfrequency of the ultrasonic transmit waveform. In addition,interpolating and filtering a Nyquist-sampled signal can result inspurious signals due to non-ideal filtering. This effect is especiallyapparent when the carrier frequency is verniered from the center of thefilter pass band.

Time interleaving multiple transmit beams is hardware efficient, but itutilizes a tradeoff between the number of transmit beams, the bandwidth,and/or the center frequency. In some implementations at the highestcenter frequency and the highest permitted bandwidth only a singletransmit beam is allowed per channel.

SUMMARY

By way of introduction, the preferred embodiment described belowcalculates ultrasonic transmit waveforms by storing a set of parametersthat defines both an envelope function and a modulation function for thedesired ultrasonic transmit waveform, and then calculating theultrasonic transmit waveform in real time based on the set ofparameters. The envelope function is preferably a smoothly rising andfalling function, such as a Gaussian function.

In this embodiment the parameters entirely define the ultrasonictransmit waveform, and for this reason the stored parameters efficientlyuse system memory, even for transmit waveforms of long duration.

The preferred transmit waveform generator comprises a transmit waveformcalculator that calculates the waveforms in the log domain, therebyminimizing the need for multipliers. Preferably, the waveform calculatorcomprises a plurality of accumulators that are operative to formrespective quadratic functions in real time. These quadratic functionsdefine the respective ultrasonic transmit waveform functions in the logor linear domain.

The disclosed embodiment combines multiple transmit waveforms usingcombiners that comprise of plurality of inputs and outputs, andmultiplexers that switch transmit waveforms from respective single orcombined transmit waveform generators to desired transducer channels.

This section is intended as a brief introduction, and it is not intendedto limit the scope of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized block diagram of an accumulator of the type usedin the preferred embodiment of this invention.

FIGS. 2a-2 k are timing diagrams used in describing the operation of theaccumulator of FIG. 1.

FIG. 3 is a block diagram of a medical diagnostic ultrasonic imagingsystem that incorporates a preferred embodiment of this invention.

FIG. 4 is a block diagram of one of the transmit waveform generators 120of FIG. 3.

FIG. 5 is a more detailed block diagram of the envelope accumulator 154and the phase accumulator 156 of FIG. 4.

FIG. 6 is a more detailed diagram of the envelope/phase processor 158 ofFIG. 4.

FIG. 7 is a circuit diagram of the envelope stage 170 of FIG. 6

FIG. 8 is a circuit diagram of the phase stage 172 of FIG. 6.

FIG. 9 is a circuit diagram of the log-linear stage 178 of FIG. 6.

FIG. 10 is a circuit diagram of the beam summer 122 of FIG. 3.

FIG. 11 is a circuit diagram of the channel summer 124 of FIG. 3.

FIG. 12 is a circuit diagram of the gain and clip stage 126 of FIG. 3.

FIG. 13 is a circuit diagram of the encoder 130 of FIG. 3.

FIGS. 14-18 are graphs used to describe the generation of a Gaussiantransmit waveform.

FIGS. 19-23 are graphs used to describe the generation of a linear FMGaussian transmit waveform.

FIGS. 24 and 25 are time and frequency domain graphs of a Hanning pulse,respectively.

FIGS. 26 and 27 are time and frequency domain graphs of a Hamming pulse,respectively.

FIGS. 28 and 29 are time and frequency domain graphs of a broadbandpulse, respectively.

FIG. 30 is a block diagram of the transmitters 102 configured in asingle-channel mode.

FIGS. 31 and 32 are block diagrams of the transmitters 102 configured ina multi-channel mode.

FIG. 33 is a plot of a waveform envelope produced by the system of FIG.32.

FIGS. 34-37 are block diagrams of single-channel (FIG. 34) andmulti-channel (FIGS. 35-37) configurations for the transmitters 102.FIG. 35 shows 128 transmit processing channels applied to a 64 elementtransducer, and FIGS. 36, 37 show 128 transmit processing channelsapplied to any contiguous block of 64 elements of a 128 elementtransducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS General Discussion

The specific examples described below generate ultrasonic transmitwaveforms efficiently by calculating quadratic functions that define theenvelope and the modulation function of the waveform. These quadraticfunctions are then combined, converted to the linear domain whereappropriate, and used to drive respective transducer elements. Thefollowing discussion presents the basic mathematical framework of theapproach implemented in the example of FIGS. 3-13.

A Gaussian ultrasonic transmit waveform x(t) can be expressed asfollows: $\begin{matrix}{{x(t)} = {{{Re}\left\{ {A\quad ^{{- {\pi {({1 - {\quad \rho}})}}}{(\frac{t - \tau}{T_{c}})}^{2}^{{2}\quad \pi \quad f}c^{({t - \tau})}}} \right\}} = {A\quad ^{- {(\frac{t - \tau}{T_{c}})}^{2}}{\cos \left( {{2\pi \quad {f_{c}\left( {t - \tau} \right)}} + {{\pi\rho}\left( \frac{t - \tau}{T_{c}} \right)}^{2}} \right)}}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$

where

A=gain,

ρ=time-bandwidth product,

T_(c)=Gaussian pulse duration,

ƒ_(c)=carrier frequency,

τ=pulse delay.

y(t), the logarithm (base 2) of the ultrasonic waveform x(t), istherefore:

y(t)=log₂(x(t))=g(t)+log₂(cos(2πθ(t))),  (Eq. 2)

and

x(t)=2^(y(t))Δ.

In Eq. 2, the function g(t) may be sampled at discrete times nΔ_(t),where Δ_(t) is the interval between samples and n is the sample number.In addition, τ may be quantized to Δ_(t) resolution such that τ is equalto n_(τ)Δ_(t). With these conventions g_(n) is equal to g(t) at timenΔ_(t), and g_(n) can be expressed as follows: $\begin{matrix}{{g_{n} = {{g\left( {n\quad \Delta_{t}} \right)} = {{{\log_{2}(A)} - {{{\pi log}_{2}(e)}\left( \frac{{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)}{T_{c}} \right)^{2}}} = {{a_{0}n^{2}} + {b_{0}n} + c_{0}}}}},} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

where $\begin{matrix}{{a_{0} = {{- {{\pi log}_{2}(e)}}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}}},} & \left( {{Eq}.\quad 4} \right) \\{{b_{0} = {2{{\pi log}_{2}(e)}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}n_{\tau}}},} & \left( {{Eq}.\quad 5} \right) \\{c_{0} = {{\log_{2}(A)} - {{{\pi log}_{2}(e)}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}{n_{\tau}^{2}.}}}} & \left( {{Eq}.\quad 6} \right)\end{matrix}$

Similarly, the function θ(t) of Eq. 2 when sampled at discrete timesnΔ_(t) may be expressed as follows: $\begin{matrix}{{\theta_{n} = {{\theta \left( {n\quad \Delta_{t}} \right)} = {{{f_{c}\left( {{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)} \right)} + {\frac{\rho}{2}\left( \frac{{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)}{T_{c}} \right)^{2}}} = {{a_{1}n^{2}} + {b_{1}n} + c_{1}}}}},} & \left( {{Eq}.\quad 7} \right)\end{matrix}$

where $\begin{matrix}{{a_{1} = {\frac{\rho}{2}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}}},} & \left( {{Eq}.\quad 8} \right) \\{{b_{1} = {{f_{c}\Delta_{t}} - {{\rho \left( \frac{\Delta_{t}}{T_{c}} \right)}^{2}n_{\tau}}}},} & \left( {{Eq}.\quad 9} \right) \\{c_{1} = {{{- f_{c}}\Delta_{t}n_{\tau}} + {\frac{\rho}{2}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}{n_{\tau}^{2}.}}}} & \left( {{Eq}.\quad 10} \right)\end{matrix}$

The embodiment of FIGS. 3-13 uses iteration techniques to generate g_(n)and θ_(n) efficiently as piecewise quadratic functions of the form

y _(n) =an ² +bn+c.  (Eq. 11)

The starting value y_(o) is set equal to c, and each subsequent valuey₁, y₂, y₃ . . . is determined iteratively according to Eq. 12:

y _(n+1) =y _(n)+T_(n) , y _(o) =c,  (Eq. 12)

where T_(o)=a+b and each subsequent value T_(n+1)=T_(n)+2 a. Theenvelope function (log domain) g_(n) and the modulation phase θ_(n) foran ultrasonic transmit waveform can be expressed as follows:

envelope (log domain): g _(n) =a ₀ n ² +b ₀ n+c ₀,  (Eq. 13)

and

modulation phase: θ_(n) =a ₁ n ² +b ₁ n+c ₁.  (Eq. 14)

The modulated envelope (log domain) y_(n) is therefore equal to

modulated envelope (log domain): y _(n) =g _(n) +LUT_(cos)[θ_(n)]=yint_(n)+yfrac_(n),  (Eq. 15)

where $\begin{matrix}{{{{LUT}_{\cos}(k)} = {\log_{2}\left( {\cos \left( {\frac{\pi}{2}k} \right)} \right)}},{k = {\left\lbrack {{0\quad \ldots \quad N_{table}} - 1} \right\rbrack/N_{table}}},} & \left( {{Eq}.\quad 16} \right)\end{matrix}$

and the modulated envelope (linear domain) {circumflex over (x)}_(n) istherefore equal to

modulated envelope (linear domain): x _(n)=2^(yint) _(^(n)) LUT_(pwr)[−yfrac_(n)],  (Eq. 17)

where $\begin{matrix}{{{{LUT}_{pwr}(k)} = 2^{- k}},\quad {k = {\frac{\left\lbrack {{0\quad \ldots \quad N_{table}} - 1} \right\rbrack}{N_{table}}.}}} & \left( {{Eq}.\quad 18} \right)\end{matrix}$

In Eq. 15 and 17, yi{circumflex over (n)}t_(n) and yfrâc_(n) representthe integer and fractional components of ŷ_(n), respectively.

FIGS. 1-2 and the following discussion explain an accumulatorimplementation of Eq. 11 and 12, and the embodiment of FIGS. 3-13provides one implementation of Eq. 13, 14, 15 and 17 that usesaccumulators like that of FIG. 1. The above describes generation of aGaussian pulse, but, by concatenating multiple fitted second ordersegments, any arbitrary transmit pulse can be generated.

Accumulator Timing

FIGS. 1 and 2 will be used to explain the timing and operation of anaccumulator 10 of the type used in the embodiment of FIGS. 3-13. Asshown in FIG. 1, the accumulator includes a memory 12 that stores a setof four parameters for each segment or zone of a selected ultrasonictransmit waveform. The accumulator 10 implements the equationy_(n)=an²+bn+c, and the four parameters for each zone include the valuesof a, b, and c for that zone as well as the value of z, the width ornumber of clock cycles of the zone.

In the following discussion, the parameters a_(n), b_(n), c_(n), z_(n)indicate the stored parameters for zone n, and the parameters a_(n,i),b_(n,i), c_(n,i) indicate calculated values for cycle i of zone n thatare used in implementing the quadratic equation for zone n defined bythe parameters a_(n), b_(n), c_(n).

Returning to FIG. 1, the parameters a_(n), b_(n), c_(n), z_(n) arestored in the memory 12 in compressed form as an integer exponent and amantissa, and they are expanded in a shift register 14. The parametersa_(n), b_(n), c_(n), z_(n), for a given zone n are read out of thememory 12 in series (FIG. 2b), and they are routed by the multiplexers16, 22, 26 and stored in the registers 18, 24, 30, respectively, duringrespective clock cycles, as shown in FIGS. 2f, 2 g, 2 h, 2 i, 2 j, and 2k. The summer 20 sums the output U of the register 18 with the output Tof the register 24, and the summer 28 sums the output S of the register30 with the output T of the register 24 (when the multiplexer 26 is inthe logic 0 state).

FIGS. 2c, 2 d, and 2 e show the values of the signals U, T and S,respectively, at various clock cycles. Using the notation of Eq. 11 and12 above, these signal values are shown in Table 1.

TABLE 1 Zone 0 (a_(o), b_(o), c_(o), z_(o) = 4) Zone 1 (a₁, b₁, c₁, z₁ >6) t i b_(o,i) c_(o,i) b_(1,i) c_(1,i) 0 0 a_(o) + b_(o) c_(o) 1 13a_(o) + b_(o) a_(o) + b_(o) + c_(o) 2 2 5a_(o) + b_(o) 4a_(o) +2b_(o) + c_(o) 3 3 7a_(o) + b_(o) 9a_(o) + 3b_(o) + c_(o) 4 0 a₁ + b₁ c₁5 1 3a₁ + b₁ a₁ + b₁ + c₁ 6 2 5a₁ + b₁ 4a₁ + 2b₁ + c₁ 7 3 7a₁ + b₁ 9a₁ +3b₁ + c₁ 8 4 9a₁ + b₁ 16a₁ + 4b₁ + c₁ 9 5 11a₁ + b₁ 25a₁ + 5b₁ + c₁ 10 6 13a₁ + b₁ 36a₁ + 6b₁ + c₁

Note that the output S of the accumulator 10 (corresponding to thevalues of c_(n,i) in Table 1) implements a piecewise quadratic equation,in which the values of S in each piecewise zone or segment aredetermined by the quadratic parameters a_(n), b_(n), c_(n) stored in thememory 12 for the zone n. The accumulator 10 is efficient to implement,because multiplication operations are not required, and the hardwareused to implement the described shifting and adding functions isrelatively simple.

Specific Examples

FIGS. 3-13 provide detailed information regarding one preferredembodiment of this invention. As shown in FIG. 3, a medical diagnosticultrasonic imaging system 100 includes a plurality of transmitters 102that supply ultrasonic transmit waveforms via amplifiers 103 and atransmit/receive switch 104 to individual transducer elements of atransducer 106. The transducer 106 forms ultrasonic pressure waves in aregion being imaged in response to these high voltage signals, andechoes from these pressure waves impinge upon the transducer 106. Theresulting echo signals are passed via the transmit/receive switch 104 toa receiver 108 that beamforms, detects and demodulates the echo signalsto form received beam signals that are processed by an image processor110 for display on a display 112.

Depending upon the application, the ultrasonic transmit waveformssupplied by the transmitters 102 may be processed (e.g. delayed, phased,apodized, gain-calibrated, delay-calibrated, and phase-calibrated) tocause the ultrasonic waves emitted by the transducer 106 to be focusedalong selected scan lines, though this is not a requirement for allembodiments. The elements 102-112 can take any suitable form, and thepresent invention is suitable for use with the widest variety of suchdevices.

Continuing with FIG. 3, each transmitter 102 includes multiple transmitwaveform generators 120, each generating a respective ultrasonictransmit waveform. Selected ones of the transmit waveforms are summed ina beam summer 122, and selected ones of the summed beams are combined ina channel summer 124. The resulting combined transmit waveforms are gaincontrolled and clipped in respective stages 126, filtered in respectivefilters 128 and encoded in respective encoders 130.

The following discussion will concentrate on the transmit waveformgenerators 120, and any suitable alternative can be used for theremaining elements 122-130, depending upon the application. The channelsummer 124, the filters 128, and the encoders 130 are optional, and maybe deleted in some embodiments. Similarly, the beam summer 122 is notrequired in all embodiments.

As shown in FIG. 4, each of the transmit waveform generators 120includes RAM memories 150, 152 for storing envelope and phaseparameters, respectively. Envelope parameters from the memory 150 areapplied to an envelope accumulator 154, and phase parameters from theRAM 152 are applied to a phase accumulator 156. The accumulators 154,156 generate envelope functions and phase functions, respectively, andthese functions are applied to an envelope/phase processor 158. Theenvelope function is in the log domain in one mode of operation. Theenvelope/phase processor 158 combines the envelope function with thephase function, in the log domain in this mode of operation, convertsthe result to the linear domain, and supplies as an output an ultrasoundtransmit waveform that is applied to the beam summer 122 of FIG. 3.

FIG. 5 provides details of construction of one preferred embodiment ofthe envelope accumulator 154 and the phase accumulator 156. Note thatthe accumulators 154, 156 in this embodiment operate as described abovein conjunction with FIGS. 1 and 2a-2 k. In this mode of operation, theoutput signal env of the envelope accumulator 154 defines the envelopeof the transmit waveform in log domain, and the output signal phs of thephase accumulator 156 defines the phase functionθ(t). The output signalsenv_zone_width and phs_zone_width correspond to the z parameterdiscussed above, and are used to control the timing at which the nextset of parameters is read out of the memories 150, 152 such that thesignals env, phs are both constructed in a piecewise manner, with eachpiece corresponding to a respective segment or zone of the ultrasonictransmit waveform in the quadratic form defined by the respective set ofstored parameters.

FIG. 6 shows a more detailed view of the envelope/phase processor 158 ofFIG. 4. The processor 158 includes an envelope stage 170 that receivesthe signal env from the envelope accumulator 154 along with apre-computed gain signal. The pre-computed gain signal may represent anapodization signal, a calibration signal, a scaling signal, or anycombination of these and other signals. Combining the various gain termsis efficient because the gain terms are simply added in the log domain.The envelope stage 170 operates in two states, depending upon the stateof the map_select signal. In a first state, the signal env is addeddirectly with the pre-computed gain signal in the log domain. In thismode the signal corresponds to a Gaussian envelope in the linear domain.In a second state, the env signal is converted from linear domain to logdomain and then added to the pre-computed gain signal. In this mode thesignal corresponds to a quadratic envelope in the linear domain. FIG. 7illustrates one preferred circuit for implementing the envelope stage170 of FIG. 6.

The processor 158 of FIG. 6 also includes a phase stage 172 thatreceives as inputs the phs signal from the phase accumulator 156 and apre-computed phase signal. The pre-computed phase signal may representcalibration values or other desired offsets for the modulation phaseangle of the transmit waveform, such as a phase adjustment toapproximate a fine delay. The phase stage 172 supplies two outputsignals: logcos, which is the log base 2 of the cosine of the sum of thesignal phs and the pre-computed phase, and sign, which is the sign oflogcos. FIG. 8 shows a circuit diagram for one preferred form of thephase stage 172.

The processor 158 of FIG. 6 includes a summer 176 that combines the logoutput of the envelope stage with the logcos output of the phase stage.In effect, the summer 176 operates in log domain to modulate theenvelope signal supplied by the envelope stage 170 with a modulationsignal supplied by the phase stage 172. The output of the summer 176 isapplied to a log-to-linear stage 178 that converts the modulatedenvelope signal in log domain supplied by the summer 176 to lineardomain. The log-to-linear stage 178 receives another input from a gate174 that defines the sign of the resulting output signal. FIG. 9 is acircuit diagram for one preferred form of the log-to-linear stage 178.

The output of the log-to-linear stage 178 is an ultrasonic transmitwaveform in linear domain that is applied as an output of the transmitwaveform generator 120.

With reference to the equations of the foregoing general discussion, theenvelope accumulator 154 of FIGS. 4 and 5 generates the signal envaccording to Eq. 13, and the phase accumulator 156 of FIGS. 4 and 5generates the signal phs according to Eq. 14. Similarly, the lookuptable LUT 0 of FIG. 8 implements Eq. 16, and the lookup table LUT 1 ofFIG. 9 implements a function closely related to that of Eq. 18.

It should be noted that the transmit waveform generator 120 calculatesthe ultrasonic transmit waveform based on a parametric description ofthe transmit waveform. The embodiment described above has been optimizedfor the calculation of linear FM modulated Gaussian pulses. In addition,this embodiment has the flexibility to generate arbitrary transmitwaveforms by fitting both the envelope amplitude and the modulatingphase angle to any desired number of piecewise second order sections.The accumulators described above are a particularly efficient hardwareimplementation. Because the computations are performed in the logdomain, all multipliers are replaced by adders and small lookup tables.This efficiency allows dedicated hardware to be used for each transmitwaveform.

The embodiment described above provides a number of importantadvantages:

1. It may be efficiently configured for modulated Gaussian transmitwaveforms, including chirps. Only eight parameters are used to defineany piecewise segment of the transmit waveform, independent of thedesired length of the segment.

2. It provides a very wide pulse bandwidth, since the sampling frequencyis at RF and filter effects can therefore be completely avoided.

3. It completely avoids extraneous aliased signal components at lowercenter frequencies, because no up-sampling techniques are required.

4. It allows high bandwidth pulses to be matched to the ideal case withexcellent accuracy.

5. As described below, up to four separate ultrasonic transmit waveformscan be combined to form a multi-beam transmit waveform, and this isindependent of the center frequency or the bandwidth that is used forindividual transmit waveforms. This is the case because the individualtransmit waveforms are generated in parallel.

6. It provides high time delay resolution for all center frequencies andit retains fine delay resolution via phasing adjustments.

7. It uses a single, highest system clock in operation and is able tomodulate to an arbitrary carrier frequency without deleterious filtereffects.

8. It uses multiple piecewise quadratic functions to approximatearbitrary envelope and phase functions.

Though less efficient, alternative embodiments of this invention can beimplemented using multipliers in the linear domain rather than addersand lookup tables in the log domain as described above.

A preferred implementation for the beam summer 122 of FIG. 3 is shown inFIG. 10. The gates 190, 192,194, 196 can be used to pass any selectedones of four ultrasonic waveform signals to the summers 198, 200, 202and thereby to the output. The output signal beamsum is thus thecombination of any one, any two, any three, or all four of the inputtransmit waveforms. The term “beam channel” will be used here to refereither to the output of one of the generators 120 or the output of thebeam summer 122.

FIG. 11 provides more detail regarding one implementation of the channelsummer 124 of FIG. 3. The channel summer in this embodiment receivesfour beam channel inputs and supplies four output signals, each destinedfor a respective transducer element. The term “transducer channel” willbe used here to refer to such output signals, at any stage along thepath from the channel summer 124 to the associated transducer element.

The channel summer 124 includes summers 210, 212, 214 that providesummation signals to multiplexers 216, 218, 220, 222. These multiplexershave three states as indicated. In state 0, each of the four inputchannels C0, C1, C2, C3 is simply applied without alteration to therespective output terminal D0, D1, D2, D3, and all of the outputterminals D0, D1, D2, D3 are active. When the multiplexers are in state1, the sum of the signals on channels C0 and C2 is applied in parallelto output terminals D0 and D2, the sum of input channels C1 and C3 isapplied in parallel to output terminals D1 and D3, and one outputterminal is active in each subset D0, D2; D1, D3. When the multiplexersare in state 2, all four of the input channels C0, C1, C2 and C3 aresummed, this sum is applied in parallel to all four of the outputterminals D0, D1, D2, D3, and only one of the output terminals D0, D1,D2, D3 is active. The channel summer 124 is an example of a combiner.Other examples include multipliers or dividers that combine two or morebeam channels, time interleavers, and time concatenators.

FIGS. 12 and 13 provide additional information regarding one preferredimplementation of the stage 126 and the encoder 130 of FIG. 3,respectively.

By way of illustration, the circuits described above for the transmitter102 can preferably be implemented in an ASIC that includes the elements120-130 for four transducer channels per package. Preferably, the outputresolution is plus or minus 256 codes, and the maximum envelope samplingrate is equal to 56 MHz. The total number of transmit waveforms per beamchannel can be varied between 1 and 4, and the maximum transmit pulselength for a real/complex envelope is greater than 8192 sampled at 56MHz.

Transmit Waveform Examples

The system described above in connection with FIGS. 3-13 can generate awide variety of transmit waveforms. This section provides a fewexamples, as well as examples for modified versions of the illustratedsystem.

a. Single-channel Gaussian Transmit Waveforms.

FIGS. 14-18 relate to a first example, in which a single transmitwaveform generator 120 generates a Gaussian transmit waveform for eachrespective beam channel. In this example, the beam summer 124 selectsonly a single generator 120 for each respective beam channel, and thechannel summer 124 passes the signal on each beam summer output directlyto the respective transducer channel.

FIG. 14 shows one example of the output signal env of the envelopeaccumulator 154 described above in conjunction with FIG. 5, for thefollowing coefficient values:

a=−0.001967;

b=0.251798;

c=−8.057529.

FIG. 15 shows one example of the output signal phs of the phaseaccumulator of FIG. 5 for the following coefficient values:

a=0.000000;

b=0.082500;

c=−4.000000.

FIG. 16 shows the resulting signals logcos and sign of FIG. 6, for thecase where precompute phase is equal to zero; and FIG. 17 shows the logdomain output of the summer 176 of FIG. 6. The resulting linear domainultrasonic transmit waveform (the beam 0 signal of FIG. 6) is shown inFIG. 18. This waveform has a Gaussian envelope with a gradually risingleading edge and a gradually falling trailing edge.

b. Single-channel, linear-FM, Gaussian Transmit Waveforms.

FIGS. 19-23 correspond to FIGS. 14-18, respectively, for a different setof quadratic coefficients. In this case, the coefficient values used forthe envelope function of FIG. 19 and the phase function of FIG. 20 areas follows:

envelope Phase a = −0.004426 a = 0.001953 b = 0.566545 b = −0.187500 c =−18.129441 c = 4.000000

The resulting logcos and sign signals are shown in FIG. 21 and theresulting sum of the envelope and modulation functions (log domain) isshown in FIG. 22. The resulting ultrasonic transmit waveform (lineardomain) is a linear-FM, Gaussian pulse, as shown in FIG. 23.

c. Single-channel Transmit Waveforms with Envelope Parameterized byFunctions Based on Cosines.

The examples described above use quadratic functions to approximate thedesired functions, but other parametric functions may be used. Forexample the envelope function env(t) of the ultrasonic waveform may beparameterized using cosine-based functions as follows: ${\begin{matrix}{{{{env}(t)} = {\sum\limits_{k = 0}^{3}{a_{k}{\cos \left( {2{\pi \cdot {k\left( \frac{t}{T} \right)}}} \right)}}}},\frac{- T}{2}} \\{{= 0},{{t} > {\frac{T}{2}.}}}\end{matrix} \leq t \leq \frac{T}{2}};$

FIGS. 24 and 25 show time and frequency domain plots, respectively, of aCOS parameterized Hanning envelope generated with the following valuesof the coefficients a_(k): 0.5, 0.5, 0, 0. FIGS. 26 and 27 show time andfrequency domain plots, respectively, of a COS parameterized Hammingenvelope generated with the following values of a_(k): 0.54, 0.46, 0, 0.FIGS. 28 and 29 show time and frequency domain plots, respectively, of aCOS parameterized broad band pulse envelope generated with the followingvalues of a_(k): 0.999448, 1.911456,1.078578, 0.183162.

d. Single-channel Modes of Operation.

In one single-channel mode of operation, the transmitter 102 assigns asingle transmit waveform generator 120 to each beam channel, and eachbeam channel is applied to a single transducer element via a singletransducer channel. FIG. 30 provides a block diagram for this mode ofoperation, using the elements of FIG. 4. In one example, the transmitwaveforms X₀(t), X₁(t) for transducer channels 0 and 1 are both pulseswith Gaussian envelopes as described above.

e. Multi-channel Modes of Operation.

In multi-channel modes of operation, the transmitter assigns multipletransmit waveform generators 120 to each active transducer channel. Forexample, the transmit waveform X₀(t) for channel 0 may take the form ofa modulated sinc function. In one embodiment, X₀(t) takes the form${X_{0}(t)} = {\frac{\sin \left( {\pi \quad {Bt}} \right)}{\left( {\pi \quad {Bt}} \right)} \cdot {{\cos \left( {2\pi \quad f_{0}t} \right)}.}}$

The logarithm (base 2) of X₀(t) is therefore expressed as follows:

log₂(X ₀(t))=log₂(sin(πBt))+log₂(cos(2πƒ_(o) t))−log₂(πBt).

This embodiment may be implemented by combining two beams as shown inFIG. 31.

This is an example of multi-channel operation in which two transmitwaveforms (each generated by a separate generator 120) overlap in time.That is, the generators 120 operate during the same time segment, andthe transmit waveform at any time during the segment is obtained bycombining transmit signals from two or more beam channels.

In another multi-channel mode of operation, two or more generators 120operate during consecutive, non-overlapping time segments to generate alonger transmit waveform. This embodiment is shown in block diagram formin FIG. 32, and the resulting transmit waveform envelope is shown inFIG. 33. Note that only the first generator 120 a associated with afirst beam channel operates during a first time segment Sa, and only thesecond generator 120 b associated with a second beam channel operatesduring the second time segment Sb. In this example, the RAMs 150,152each hold only three zones of parameters, but the overall transmitwaveform has six separate zones: three zones generated by the generator120 a and three zones generated by the generator 120 b.

FIGS. 34-37 illustrate several modes of operation for the embodiment ofFIG. 3, which for purposes of discussion is assumed to have 128 beamchannels and 128 transducer channels. In this example, the outputs ofthe beam summer 122 will be referred to as respective beam channels, andthe outputs of the channel summer 124 will be referred to as transducerchannels. In the mode of FIG. 34, beam channels 0-63 are used to drivetransducer elements 0-63 of a 64-element transducer, and beam channels64-127 are idle. This is an example of single-channel operation.

In the mode of FIG. 35, beam channels n and (n+64) are combined to drivetransducer element n via transducer channel n (0≦n≦63). This is anexample of multi-channel operation.

FIGS. 36 and 37 provide two examples of multi-channel operation for thecase where 128 beam channels are used with a 128-element transducer. InFIG. 36, only elements 0-63 are active, and each element n is coupledwith two beam channels: n and (n+64). In FIG. 37, only elements 64-127are active, and each element (n+64) is coupled with two beam channels: nand (n+64). Any contiguous block of 64 elements can be driven(simultaneously) by two beam channels per element.

The channel combiners 124 of FIGS. 35-37 may combine the respective beamchannel signals in many ways, including by adding, multiplying ordividing the signals on the respective beam channels.

Thus, a single-channel waveform is determined as a function of theprocessing capability of a single beam channel, and a multi-channelwaveform is determined as a function of the processing capability of twoor more beam channels. The embodiments of FIGS. 35-37 provide theadvantage that the processing power of beam channels that wouldotherwise be idle is used to form more complex or longer transmitwaveforms on the active transducer channels.

Of course, it should be understood that many changes and modificationscan be made to the preferred embodiments described above. For example,the parametric waveform generation techniques described above can beimplemented with multipliers rather than accumulators, and the channelsumming and beam summing techniques described above can be used withother sources of ultrasonic transmit waveforms. The term “source” asused in this context is intended broadly to encompass any source ofultrasonic transmit waveforms, including those generated directly frommemory, and those generated by modulating a stored or interpolatedenvelope using signal processing techniques, for example.

In an alternative embodiment, the ultrasonic transmit waveform iscalculated in real time from a set of parameters that define thewaveform directly, rather than defining the waveform as an envelopefunction that is modulated by a modulation function. Orthogonalfunctions such as Walsh functions and Hermite functions are examples offunctions that may be used.

As used herein, the term “ultrasonic transmit waveform” is intended torefer to an RF frequency ultrasonic waveform that is applied to atransducer, at any stage in the signal path between the output of thetransmit waveform generator 120 and the input to the transducer 106. Theterm “transmit waveform” is used to refer either to a piece or a zone ofa total pulse, or the entire pulse.

The term “combiner” is intended broadly to include summers, multipliers,lookup tables and the like, whether operating in parallel or intime-interleaved fashion.

The term “calculate” is intended to include calculation in both the logdomain and the linear domain, but to exclude signal processingtechniques operating on sampled baseband envelopes.

The term “set” is used broadly to encompass one or more, and the term“accumulator” is used broadly to encompass adders. The term “logarithm”or “log” is intended to encompass logarithms in any base.

The foregoing detailed description has discussed only a few of the manyforms that this invention can take. For this reason, this detaileddescription is intended by way of illustration and not limitation. It isonly the following claims, including all equivalents, that are intendedto define the scope of this invention.

What is claimed is:
 1. In a medical ultrasound imaging system, a methodfor generating a transmit waveform comprising: (a) storing a set ofparameters that defines both an envelope function and a modulationfunction for an ultrasonic transmit waveform having more than two peakamplitude levels; and (b) calculating the ultrasonic transmit waveformin real time based on the set of parameters.
 2. The method of claim 1wherein the envelope function for the ultrasonic transmit waveform isnon-rectangular.
 3. The method of claim 1 wherein the envelope functionfor the ultrasonic transmit waveform gradually rises during an initialportion of the ultrasonic transmit waveform.
 4. The method of claim 1wherein the envelope function for the ultrasonic transmit waveformgradually falls during a final portion of the ultrasonic transmitwaveform.
 5. The method of claim 1 wherein the set of parameters definesthe envelope function in log domain.
 6. The method of claim 5 whereinthe envelope function comprises at least one quadratic envelopefunction, and wherein the set of parameters comprises at least one setof quadratic envelope coefficients.
 7. The method of claim 1 wherein themodulation function comprises at least one quadratic phase function, andwherein the set of parameters comprises at least one set of quadraticmodulation coefficients.
 8. The method of claim 1 wherein (a) comprisesstoring the set of parameters in a digital memory.
 9. The method ofclaim 8 wherein (b) comprises calculating the transmit waveform with adigital processor.
 10. The method of claim 1 wherein the envelopefunction comprises a plurality of quadratic envelope functions, eachassociated with a respective zone of the transmit waveform, and whereinthe set of parameters comprises a plurality of respective sets ofquadratic envelope coefficients.
 11. The method of claim 1 wherein themodulation function comprises a plurality of quadratic phase functions,each associated with a respective zone of the transmit waveform, andwherein the set of parameters comprises a plurality of respective setsof quadratic phase coefficients.
 12. A transmit waveform generator for amedical ultrasound imaging system, said transmit waveform generatorcomprising: (a) a memory storing a set of parameters that defines bothan envelope function and a modulation function for an ultrasonictransmit waveform having more than two peak amplitude levels; and (b) atransmit waveform calculator operative to calculate the transmitwaveform in real time based on the set of parameters stored in thememory.
 13. The invention of claim 12 further comprising a set ofdigital to analog converters responsive to the transmit waveforms. 14.The invention of claim 12 wherein the set of parameters defines theenvelope function in log domain.
 15. The invention of claim 12 whereinthe set of parameters defines the envelope function in linear domain.16. The invention of claim 14 wherein the envelope function comprises atleast one quadratic envelope function, and wherein the set of parameterscomprises at least one set of quadratic envelope coefficients.
 17. Theinvention of claim 12 wherein the modulation function comprises at leastone quadratic phase function, and wherein the set of parameterscomprises at least one set of quadratic phase coefficients.
 18. Theinvention of claim 12 wherein each memory stores the respective set ofparameters in digital form.
 19. The invention of claim 18 wherein eachcalculator comprises a respective digital processor.
 20. The inventionof claim 12 wherein the envelope function comprises a plurality ofquadratic envelope functions, each associated with a respective zone ofthe transmit waveform, and wherein the set of parameters comprises aplurality of respective sets of quadratic envelope coefficients.
 21. Theinvention of claim 12 wherein the modulation function comprises aplurality of quadratic phase functions, each associated with arespective zone of the transmit waveform, and wherein the set ofparameters comprises a plurality of respective sets of quadratic phasecoefficients.
 22. The transmit waveform generator of claim 12 whereinthe transmit waveform calculator is operative to calculate in a logdomain and in realtime a first signal indicative of a logarithm of atleast a component of the ultrasonic transmit waveform; and furthercomprising: a converter responsive to the calculator and operative toconvert a signal that varies as a function of the first signal to asecond signal indicative of the ultrasonic transmit waveform in lineardomain.
 23. The invention of claim 22 wherein the calculator comprises asummer operative in the log domain to effect apodization multiplicationin the linear domain.
 24. The invention of claim 22 wherein thecalculator comprises a summer operative in the log domain to effecttransmit gain multiplication in the linear domain.
 25. The invention ofclaim 22 wherein the calculator comprises a summer operative in the logdomain to effect envelope and modulation multiplication in the lineardomain.
 26. The invention of claim 22 wherein the calculator comprises asummer operative in the log domain to effect gain calibrationmultiplication in the linear domain.
 27. The invention of claim 22wherein the calculator comprises a summer operative in the log domain toeffect multiplication in the linear domain.
 28. The invention of claim22 wherein the calculator calculates the logarithm in log base
 2. 29.The transmit waveform generator of claim 12 wherein the transmitwaveform calculator comprises a plurality of accumulators, eachaccumulator operative to generate a respective output signal in a logdomain and in real time, each output signal indicative of a logarithm ofa respective component of the ultrasonic transmit waveform; and furthercomprising: a processor operative to combine the output signals in thelog domain to form a combined signal.
 30. The invention of claim 29further comprising: a converter responsive to the combined signal andoperative to convert the combined signal to linear domain.
 31. Theinvention of claim 29 wherein the calculator implements a quadraticfunction.
 32. The transmit waveform generator of claim 12 wherein thetransmit waveform calculator comprises at least one accumulatorresponsive to a plurality of polynomial coefficients to generate apolynomial function in real time, the polynomial function indicative ofat least a component of the ultrasonic transmit waveform.
 33. Theinvention of claim 32 wherein the polynomial function is indicative ofat least the component of the ultrasonic transmit waveform in logdomain.
 34. The invention of claim 32 wherein the polynomialcoefficients comprise quadratic coefficients, and wherein the polynomialfunction comprises a quadratic function.
 35. The invention of claim 33wherein the polynomial coefficients comprise quadratic coefficients, andwherein the polynomial function comprises a quadratic function.
 36. In amedical ultrasound imaging system, a method for generating a transmitwaveform comprising: (a) storing a set of parameters that entirelydefine an ultrasonic transmit waveform having more than two peakamplitude levels; (b) generating the ultrasonic transmit waveform inreal time based on the set of parameters.
 37. The method of claim 36wherein the set of parameters comprises envelope parameters that definean envelope function for the ultrasonic transmit waveform.
 38. Themethod of claim 37 wherein the envelope parameters comprise polynomialcoefficient parameters.
 39. The method of claim 37 wherein the envelopeparameters comprise coefficients of a set of base functions associatedwith the envelope function.
 40. The method of claim 36 wherein the setof parameters comprises modulation parameters that define a modulationfunction for the ultrasonic transmit waveform.
 41. The method of claim40 wherein the modulation parameters comprise polynomial coefficientparameters.
 42. The method of claim 40 wherein the modulation parameterscomprise coefficients of a set of base functions associated with themodulation function.
 43. The method of claim 36 wherein the set ofparameters comprises at least first parameters associated with a firstsegment of the ultrasonic transmit waveform and second parametersassociated with a second segment of the ultrasonic transmit waveform,and wherein (b) comprises generating the ultrasonic transmit waveform ina piecewise manner.
 44. The method of claim 43 wherein (b) comprisesgenerating an envelope function for the ultrasonic transmit waveform ina piecewise manner.
 45. The method of claim 43 wherein (b) comprisesgenerating a modulation function for the ultrasonic transmit waveform ina piecewise manner.
 46. The method of claim 36 wherein the set ofparameters defines the ultrasonic transmit waveform directly.
 47. Themethod of claim 46 wherein the parameters comprise coefficients of a setof base functions.